Phenotypic assortative mating and within-pair sexual dimorphism and its influence on breeding success and offspring quality in Magellanic penguins

We examined within-pair sexual dimorphism and phenotypic assortative mating in Magellanic penguins (Spheniscus magellanicus) breeding in six colonies located on the Patagonian coast (Argentina). All measured phenotypic traits except the number of pectoral spots differed between the sexes; bill depth and flipper length were the most and least dimorphic traits, respectively. We found assortative mating by bill depth and body mass. The similarity in body condition within pairs was close to significant. When we performed se parate correlations for birds that bred successfully, i.e., raised one or t wo offsprings, and birds that did not attempt to breed or bred unsuccessful ly, only the successful breeders showed assortative mating by body mass. In addition, we attempted to relate the body size of each member of the pair and the degree of sexual dimorphism within pairs to the breeding performanc e of individuals, which was measured as brood size, and body condition and immunocompetence of offspring. We found that pairs that were less dimorphic in flipper length raised more offspring. This effect was due to female fli pper length per se and not to the relative difference in flipper length bet ween members of the pair. Females with larger flippers had a higher probabi lity of raising two chicks. No effects of body measurements or degree of se xual dimorphism on body condition or T-cell-mediated immune response of off spring were found. We discuss these results in the context of potential fac tors responsible for the maintenance of sexual size dimorphism in this spec ies.